USE INTRODUCTION 

The Phenomenon of osmosis (from the Greek for `push’) is the tendency of a pure solvent to enter a solution separated from it by a semi permeable membrane (a membrane permeable to the solvent by not to the solute). The flow can be opposed by applying pressure to the solution, and the osmotic pressure is the pressure that just stops the flow. When the applied pressure exceeds the osmotic pressure, the movement of solvent can be reversed, i.e., flow of solvent from solution to pure solvent. This process is known as reverse osmosis (RO), and is useful for generating pure water from saline water.

There are five important characteristics of  semi permeable membranes which determine their quality, namely. 

• Salt rejection efficiency
• Flux
• Tolerance to temperature, pH variations, chemical constituents
• Fouling tendency
• Durability

Thin film composite (TFC) RO Membranes have important advantages over the previous generation cellulose acetate (CA) asymmetric membranes as developed by this Institute. These are indicated in Table 1.

TABLE 1 

It can be appreciated that the relative importance of some of the parameters depends on the specific application.  For example, where seawater desalination through a single pass is required, salt rejection efficiency is critical.  On the other hand, for brackish water with Total Dissolved Solids (TDS) < 10,000 ppm, a salt rejection efficiency of the order of 95% would yield water conforming to WHO specifications. Indeed, too high a rejection efficiency would reduce mineral content of the water to unacceptably low levels requiring subsequent remineralisation and is therefore undesirable.  What is more important in such application is the requirement of high flux to minimize capital and operational costs. Besides, rugged operation and durability are also very important especially for applications in rural areas.

TFC membrane comprises a fabric, a polysulfone film, and an ultra thin salt rejecting barrier coating. A further protective coating on top of the barrier layer is also desirable. The membrane is prepared in successive steps of (i) solution casting of polysulfone on the fabric, (ii) interfacial in situ polymerization of the polyamide barrier layer on the polysulfone, and (iii) introduction of the protective layer. TFC membrane preparation has been considered to be complex, requiring sound understanding of the influence of different process variables on performance and a high degree of precision-based technology. As such, only a few countries such as U.S.A. and Japan have so far been able to master this type of technology for desalination. TFC membranes and modules for desalination of brackish water, dewatering applications, and treatment of industrial/domestic effluents for water reuse have now been developed at CSMCRI, Bhavnagar. The modules have been tested extensively both in the laboratory and in the field and the performance has been satisfactory and consistent.

The principal innovations in the development of the technology are as follows :

MEMBRANE

MODULE

 Some patents for the above inventions have already been secured and a few additional patent applications are being filed : 

1.      “Design and Development of Membrane Casting Machine”, Indian Patent No. 145945.

2.      “A Process for the Preparation of High Flux-High Separation Thin Film Composite Reverse Osmosis Membrane for Desalination of Highly Saline Waters” (Indian Patent No. 169550).

APPLICATION AREAS 

There are five primary areas where RO technology can be used. These are (i) Producing potable water (ii) treatment of saline industrial effluent, (iii) treatment of tertiary treated sewage water and reuse, (iv) dewatering and concentration, and (v) ultra pure water. These application areas are discussed in more detail below :

 Potable Water 

With regards to improvement in water quality, there is just no other process presently that can compete with RO.  Reverse osmosis technique for water treatment is highly effective in solving several problems simultaneously: 

(a)  Salinity can be reduced by more than 95%

(b)  Water becomes crystal clear

(c)   The water is free of bacteria

(d)  There is reduction in hardness

(e)  Other harmful constitutes in water such as fluoride, arsenic and heavy metals can be removed to the extent of 90% or more. 

In short, water is purified, desalinated and softened at one go.  Not surprisingly many plants producing bottled water employ RO in the purification step. Table 2 provides relevant data for a 30,000 lpd plant installed by the Institute at Mocha Village, near Porbandar, Gujarat.  CSMCRI’s  brackish water desalination technology also allows for a constant TDS range (250-500 ppm) of product water TDS level of 10,000 ppm. CSMRI has installed several plants (5000-30000 litres/day) based on its TFC membrane technology to cater to societal needs and meeting the drinking water requirements such as in the aftermath of the Orissa super cyclone, in the recent earthquake hit region of Gujarat and in certain drought prone villages of Saurashtra suffering from acute problem of brackishness.  More recently, the institute has developed a domestic RO unit to cater to the drinking water needs of a small family. 

INDUSTRIAL EFFLUENT TREATMENT 

Treatment of saline industrial effluent has become a major challenge. Such effluent are generated by tanneries, dyeing houses and cellulose manufacturers.  Most such effluents are in the range of 8000-10000 ppm and, therefore, would be amendable to treatment by CSMCRI’s brackish water membrane.  The primary role of treatment would be to recycle 65-75% water. However, it must be pointed out that disposal of the saline reject effluent poses a major challenge in all such treatment processes. The Institute has developed a cultivation technology which utilizes the saline effluent for irrigation but this approach can have only limited applicability.  In some situations it may be possible to recycle the reject effluent in the manufacturing process or, alternatively, the effluent can be discharge dint eh sea where feasible or charged into solar salt works. It is also possible to devise methods of converting the saline reject into solid waste or regenerate acid and alkali from the salt with bipolar ED membrane technology. Bipolar ED would be an attractive option for the cellulose industry, as the acid and alkali can be recycled in the cellulose manufacturing process but the concept remains to be popularized. 

TREATMENT OF TERTIARY TREATED SEWAGE WATER 

Tertiary treated domestic sewage effluent is also being treated by RO, e.g., at Chennai Petrochemicals Limited, and CSMCRI is in the process of installing a 1 million litres/day plant based on its indigenous membrane technology. 

DEWATERING AND CONCENTRATION

RO is an excellent  method of concentration of chemicals in aqueous solutions at ambient temperature. This is because the membrane only allows water to pass through but not the other chemical entities.  This is used in some pharmaceutical industries. CSMCRI has also recently fabricated an aqueous herbal extract concentrator that utilizes its indigenous membrane. RO technology is also used overseas for other dewatering application such as in the dairy and food product industry. 

ULTRAPURE WATER 

Ultrapure water is required in almost all research laboratories besides in electronic industries and other industries where water purity of high grade is essential.  RO is the principal component of such purification units. Water of sufficient purity is also required as Boiler Feed Water. 

OTHER APPLICATIONS 

TFC RO membrane technology involves intermediate production of polysulfone membrane.  This serves as an ultrafiltration membrane and is useful in a number of applications such as removal of pathogenic bacteria from, otherwise sweet water (this is an attractive alternative to boiling, chlorination, ozonolysis and UV radiation) or as part of a pretreatment system to RO. The porosity of the polysulfone membrane can be controlled and such technology would be of considerable interest for fractionation of chemicals of varying molecular weight. 

INFRASTRCUTURAL REQUIREMENTS 

Building Requirement 

About 1000 sq. m. carpet area will be required of which 30% should be air conditioned. 

Facilities/equipment requirement for membrane production 

• Chemical laboratory with heating mantle, stirrers, ovens, conductivity meters, pH meters, balance etc.
• Polysulfone casting machine
• Thin film forming and curing machine
• Membrane protective layer forming machine
• Membrane quality control test kits
• Analytical instruments recommended for membrane quality control : FT-IR, Bubble porosimeter, GPC, GC, HPLC
• Power generating set, water chilling machine, dehumidifiers.

The estimated cost would be Rs.175 lakhs. 

Chemical & consumables required for making TFC membrane 

• Non-woven polyester fabric as a base material.
• Polymers such as polysulfone, polyether sulfone.
• Solvents such as dimethyl formamide, dimethylsulfoxide, dimethylacetamide, n-hexane, n-heptane.
• Diamines such as metaphenylene diamine, polyethylenimine, piperazine.
• Acid chlorides such as trimesoly chloride, isophthaloyl chloride, terephthaloyl chloride.
• Chemicals like polyvinyl alcohol, glycerine and PEG.

Production Volume 

5000 sq. m./month of RO membrane can be produced working in two shifts. 

Membrane production cost 

The variable cost of membrane manufacture would be ~ Rs.200/sq m which includes the following components. 

Chemicals and Consumables                              Rs.160/sq. m.

Energy Cost                                                   Rs. 40/sq. m.

Production-related manpower cost     Rs. 20/sq. m.

(3 B.Sc. chemists + 1 mechanic + 4 helpers per shift _ 1 Chem Engg.)

Manufacture of TFC spiral elements 

Facilities required 

• Module making facility comprising (i) membrane spiral making machine, (ii) membrane cutting device, (iii) membrane rolling machine (iv) spiral element trimming machine, (v) product tube making machine, (vi) end caps moulding machine, (vii) outer hard wrapping machines, (viii) sonic welding machine, (ix) permeate and feed spacer automatic cutting machine.
• Spiral element test benches, water filters, chemical dosing, circulating pumps, high pressure pumps, etc.

The estimated cost would be Rs.75 lakhs. 

Materials required for making TFC spiral modules 

• TFC membrane
• Melamine impregnated knitted spacer
• LDPE net feed spacer
• Adhesives
• Product tube, end cap, fibre glass cloth.

Production volume

Production of modules utilizing 5000 sq m of TFC membrane per month. For example, five hundred 4” diameter x 1 m spiral elements or six thousand 2” x 12” domestic RO cartridges can be made from this quantity of membrane. 

Module Cost 

The variable cost of production of a 4” diameter TFC spiral module will be around Rs.6100 which includes the following components : 

10 sq.m. TFC membrane                                           Rs.2,200

Product and feed spacers ( 5 sq m each)                    Rs.2,000*

Product tube (1.2 m), end caps, adhesive, hard wrap    Rs.1,600*

Energy cost                                                           Rs.50

Labour cost (7 semi-skilled/shift x 2 shifts)                  Rs.250           

            *There is scope to reduce cost through indigenisation.  

Brackish water RO Plant based on indigenous Thin Film Composite (TFC) Membrane Technology 

Feed water of up to 10,000 ppm TDS can be treated; Excludes cost of shed, storage tanks, water source with pipeline, pipeline for reject water disposal; Operating cost includes: power (Rs.4/unit basis), operator wages, chemicals, equipment maintenance, and membrane replacement (30% replacement/year).